Theiler's disease is a fulminant hepatic disease affecting horses. The cause of Theiler's disease has not yet been elucidated and represents a significant problem related to the diagnosis and treatment of the disease. The disease has been associated with the antecedent administration of biologics of equine origin (e.g., botulinal antitoxin, tetanus antitoxin). A significant delay between exposure to biologics and the onset of acute hepatitis has been observed and the associated histopathologic lesions in the liver determined to be remarkably similar to those associated with acute human hepatitis A virus or hepatitis B virus infection.